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FIVE WAYS TO MANAGE THE LOAD To size a generator, you need to know the load of the things you want to power. You can find the power requirements for an appliance on the body of the device, on the nameplate, or on the data tag on the electric motor. If the draw is listed in amps, you can convert to watts with this formula: watts = amps x volts. Most items run on 120v, but dryers, furnaces, and some other appliances run on 240v. Once you know the loads, add them up. There’s a catch, though: You have to account for start-up loads. Electric motors need more energy to start than they do once they are running. For example, a refrigerator might consume 540w when running, but it’ll draw 1200w for a few seconds after the compressor has started. You don’t typically run all those loads at the same time, so you don’t need to meet the total starting load. Instead, you either build in extra capacity for these starting loads, or you manage them by turning off other circuits when large electric motors start. This can be done manu- THE POWER AND PRICE CONTINUUM 5000w ally by switching circuit breakers or with the help of a transfer switch programmed to direct power to some circuits and shunt other circuits based on the total load. Why not install a larger generator instead of using load- managing transfer switches to keep demand under the generator’s output? The answer is cost. For standby generators between 10kw and 20kw, the cost at the margin for each increase of 1000w is relatively small. Once you exceed 20kw, however, generators require a liquid-cooled engine rather than an air-cooled engine. This upgrade—like jumping from a lawn-mower engine to a car engine—doubles or triples the price. The price listed with each category below is only a starting point. Expect the actual cost to vary greatly depending on the size of the house, the size of the load, and the specific transfer switch used. Also, installation costs are not included and can easily rival the cost of the generator and the transfer switch. Portable generators 10kw Standby generators 1 Lowest cost, least convenience Based on the typical load of a 2500-sq.-ft. house, a 5500w or larger portable generator offering 240v electricity and paired with a manual transfer switch can power critical loads. A good rule of thumb is to have one circuit for each kilowatt of generator power (excluding electric heat and AC loads). A 6000w generator with a six-circuit transfer switch could back up a sump pump, a freezer, and a refrigerator, with two or three circuits left for lights and a TV, and possibly another kitchen circuit for a microwave. It’s unlikely that all these devices would be on at once, so with care, you could connect additional circuits and power them intermittently by manually turning circuits on and off. $1000 48 FINE HOMEBUILDING 2 Automatic essential power For the same home in option 1, a permanently installed 10kw generator and 12circuit subpanel-type automatic transfer switch mean you don’t have to connect and start the generator manually, or refuel it during use. The 12 circuits are the only circuits powered, but you’ll be able to add a well pump, an electric water heater, and possibly a small (3-ton) air conditioner to the load. $3000 3 Doing more with less capacity The largest air-cooled generator (20kw) might back up the whole house in the 2500-sq.-ft. scenario as long as the central air-conditioning compressor isn’t more than 5 tons and the house doesn’t have electric heat. However, even a slightly larger house could have two AC compressors. They might use 7800w running together, but their combined start-up draw would be 22,800w— enough to overload a larger generator. Generac’s load-managing Nexus Smart Switch (about $700) has two dedicated circuits for air conditioners that stagger their starts. This reduces the required size of the generator and saves you from spending two to three times more for a liquid-cooled model to handle the unmanaged start-up load of two AC units. You also can add up to four modules—$60 devices installed along the circuit’s wiring—that will monitor the load and, if necessary, delay and stagger the demands of additional energy hogs like well pumps, sump pumps, and electric ranges. $5000